1. Field of the Invention
This invention relates in general to a method for forming isolation, and more specifically relates to a method for forming shallow trench isolation (STI) which effectively reduces the kink effect.
2. Description of Related Art
There may be hundreds of thousands of devices formed on a single silicon substrate within an area of about 1-2 cm.sup.2. In order to keep the devices from disturbing each other, the existence of isolation regions formed between the devices is very important for modern semiconductor manufacturing technology. The local oxidation of silicon (LOCOS) is used widely for forming isolation regions in a conventional method. Another method for forming isolation regions is shallow trench isolation (STI,) by which a trench is formed within a substrate and is filled with insulator. The STI method is a necessary technology for process under 0.25 .mu.m.
FIGS. 1A through 1D schematically illustrate a conventional manufacturing process for forming a shallow trench isolation (STI) structure. Referring to FIG. 1A, a pad oxide 11 and a mask layer 12 are sequentially formed on a semiconductor substrate 10 and are patterned by photolithography and etching technologies. A trench 13 inside he substrate 10 is formed by anisotropic etching.
Referring to FIG. 1B, a liner oxide 14 is formed along the sidewalls and the bottom of the trench 13. Afterwards, an insulation layer is deposited on the mask layer 12 and fills the trench 13. With the help of chemical mechanic polishing (CMP), the insulation layer outside the trench 23 is removed and an insulation plug 15 is formed thereafter, as shown in FIG. 1C. A shallow trench isolation structure is then formed as shown in FIG. 1D after the mask layer 12 and the pad oxide 11 are removed.
Because the step for removing the pad oxide 11 uses isotropic etching, an over etching effect easily occurs at the edges of the insulation plug 15 by which a narrow dished portion 19 is formed and exposes a sharp corner 16. The sharp corner 16 easily accumulates electric charges, which will reduce the threshold voltage of devices to produce an abnormal sub-threshold current, called the kink effect. Therefore, the quality and the yield of devices are reduced.
At present time, the method mostly adapted for improving the kink effect is to change the corner shape so as to reduce the sub-threshold current. Rounding the sharp corner is a method for reducing the kink effect, and a rounded corner 17 is shown in FIG. 1E. Another conventional method for reducing the kink effect is to fill the narrow dished portion with insulator. However, the sub-threshold current is still produced even when the corner shape is changed. A large amount of the electric charge is induced by plasma used in the following processes, which the induced charges accumulate around the corner of the trench so that the kink effect occurs again.